Low temperature nano particle preparation and deposition for phase-controlled compound film formation

ABSTRACT

The present invention is directed to different methods used in the formation of an ink, as well as being directed to the formation of layers used in the fabrication of a solar cell, particularly the absorber layer. In one embodiment, the invention is directed to formulating an ink comprising Cu-rich particles and solid Ga—In particles, wherein the step of formulating is carried out at a temperature such that no liquid phase is present within the solid Ga-In particles. In another embodiment, the specific steps taken during the formulation of the ink are described. In yet another embodiment, the process of using the formulated ink to obtain a precursor layer are described.

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/474,259 filed May 19, 2004 and U.S. patent application Ser.No. 11/070,835 filed Mar. 1, 2005, each of which are expresslyincorporated by reference herein, and also claims priority from U.S.Provisional Appl. Ser. No. 60/567,459 filed May 4, 2004, U.S.Provisional Appl. Ser. No. 60/283,630 and U.S. Provisional Appl. Ser.No. 60/60/548,297, all of which are also expressly incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to method and apparatus for preparing thinfilms of semiconductors and more specifically growing of compoundsemiconductor films as absorber layers for solar cell structures.

BACKGROUND

Solar cells are devices that convert sunlight directly into electricalpower. The most common solar cell material is silicon, which is in theform of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce cost ofsolar cells for terrestrial use. One way of reducing the cost of solarcells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se,Te, Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1-x)Ga_(x) (S_(y)Se_(1-y))₂ (where0≦x≦1and 0≦y≦1) have already been employed in solar cell structures thatyielded conversion efficiencies approaching 20%. Absorbers containingGroup IIIA element Al and/or Group VIA element Te also showed promise.Therefore, in summary, compounds containing: i) Cu from Group IB, ii) atleast one of In, Ga, and Al from Group IIIA, and iii) at least one of S,Se, and Te from Group VIA, are of great interest for solar cellapplications.

In a thin film solar cell employing a Group IBIIIAVIA compound absorber,the cell efficiency is a strong function of the molar ratio of IB/IIIA.If there are more than one Group IIIA materials in the composition, therelative amounts or molar ratios of these IIIA elements also affect theproperties. For a Cu(In,Ga)(S,Se)₂ absorber layer, for example, theefficiency of the device is a function of the molar ratio of Cu/(In+Ga).Furthermore, some of the important parameters of the cell, such as itsopen circuit voltage, short circuit current and fill factor vary withthe Ga/(Ga+In) molar ratio. In general, for good device performanceCu/(In+Ga) molar ratio is kept at or below 1.0. As the Ga/(Ga+In) molarratio increases, on the other hand, the optical bandgap of the absorberlayer increases and therefore the open circuit voltage of the solar cellincreases while the short circuit current typically may decrease. It isimportant for a thin film deposition process to have the capability ofcontrolling both the molar ratio of IB/IIIA, and the molar ratios of theGroup IIIA components in the composition. It should be noted thatalthough the chemical formula is written as Cu(In,Ga)(S,Se)₂, a moreaccurate formula for the compound is Cu(In,Ga)(S,Se)_(k), where k istypically close to 2 but may not be exactly 2. For simplicity we willcontinue to use the value of k as 2. It should be further noted that thenotation (X,Y) in chemical formula means all chemical compositions of Xand Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example,Cu(In,Ga) means all compositions from CuIn to CuGa.

The first technique that yielded high-quality Cu(In,Ga)Se₂ films forsolar cell fabrication was co-evaporation of Cu, In, Ga and Se onto aheated substrate in a vacuum chamber. This technique is still popular interms of growing absorber layers that yield high conversion efficienciesin thin film solar cell structures. However, low materials utilization,high cost of equipment, difficulties faced in large area deposition andrelatively low throughput are some of the challenges faced incommercialization of the co-evaporation approach.

Another technique for growing Cu(In,Ga)(S,Se)₂ type compound thin filmsfor solar cell applications is a two-stage process where at least twocomponents of the Cu(In,Ga)(S,Se)₂ material are first deposited onto asubstrate, and then reacted with each other and/or with a reactiveatmosphere in a high temperature annealing process. For example, forCuInSe₂ growth, thin layers of Cu and In are first deposited on asubstrate and then this stacked precursor layer is reacted with Se atelevated temperature. If the reaction atmosphere also contains sulfur,then a CuIn(S,Se)₂ layer can be grown. Addition of Ga in the precursorlayer allows the growth of a Cu(In,Ga)(S,Se)₂ absorber.

There are many other versions of the two-stage process that have beenemployed by different research groups. For example, stacked layers ofsputter deposited (Cu—Ga)/In, and co-evaporated (In—Ga—Se)/(Cu—Se), and(In—Ga—Se)/Cu stacks have all been used as precursor materials whichwere reacted at high temperatures with S and or Se to form the finalabsorber film. In two-stage processes individual thicknesses of thelayers forming the stacked structure are controlled so that the twomolar ratios mentioned before, i.e. the Cu/(In+Ga) ratio and Ga/(Ga+In)ratio, can be kept under control from run to run and on large areasubstrates.

Sputtering or evaporation techniques have been used in prior artapproaches to deposit the layers containing the Group IB and Group IIIAcomponents of the precursor stacks. In the case of CuInSe₂ growth, forexample, Cu and In layers were sputter-deposited on non-heatedsubstrates and then the composite film was selenized in H₂Se gas or Sevapor at an elevated temperature, as described in U.S. Pat. No.4,798,660. More recently, Ga was introduced into the compound film byincluding Ga in the sputter deposited precursor stack. Such techniquessuffer from high cost of capital equipment, and relatively slow rate ofproduction as well as difficulties to control individual thicknesses oflayers.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1. The device 10 is fabricated on a substrate 11, such as of asheet of glass, a sheet of metal, an insulating foil or web, or aconductive foil or web. The absorber film 12, which comprises a materialin the family of Cu(In,Ga,Al)(S,Se,Te)₂, is grown over a conductivelayer 13, which is previously deposited on the substrate 11 and whichacts as the ohmic contact to the device. Various conductive layerscomprising Mo, Ta, W, Ti, stainless steel etc. have been used in thesolar cell structure of FIG. 1. If the substrate itself is a properlyselected conductive material, it is possible not to use a conductivelayer 13, since the substrate 11 may then be used as the ohmic contactto the device. After the absorber film 12 is grown, a transparent layer14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film.Radiation 15 enters the device through the transparent layer 14.Metallic grids (not shown) may also be deposited over the transparentlayer 14 to reduce the effective series resistance of the device. Thepreferred electrical type of the absorber film 12 is p-type, and thepreferred electrical type of the transparent layer 14 is n-type.However, an n-type absorber and a p-type window layer can also beutilized. The preferred device structure of FIG. 1 is called a“substrate-type” structure. A “superstrate-type” structure can also beconstructed by depositing a transparent conductive layer on atransparent superstrate such as glass or transparent polymeric foil, andthen depositing the Cu(In,Ga,Al)(S,Se,Te)₂ absorber film, and finallyforming an ohmic contact to the device by a conductive layer. In thissuperstrate structure light enters the device from the superstrate side.

A variety of materials, deposited by a variety of methods, can be usedto provide the various layers of the device shown in FIG. 1. Thesedeposition techniques are well known in the field and will not berepeated here. The present invention concerns itself more with thegrowth of the absorber film 12, which is the heart of the solar cellstructure.

As reviewed above, vacuum processes such as co-evaporation andsputtering are expensive techniques. With a goal of finding aninexpensive approach to absorber layer fabrication, research groups haveinvestigated techniques comprising the steps of: i) preparing aprecursor in the form of an ink or slurry containing particles orpowders of all or some of the components of Cu(In,Ga)(S,Se)₂ compound,ii) depositing the ink or slurry on a substrate using methods such asspraying, doctor-blading and screen printing, to form a precursor layercomprising particles of the ink, and iii) reacting the particle orpowder-based precursor layer at elevated temperatures typically with Seand/or S to from the compound film. In the formation of the inks orslurries well known agents such as liquid carriers, dispersants,surfactants etc. were used in conjunction with the powders to form wellbehaved solutions that could be coated on substrates in thin film form.

Some of the above particle-based approaches utilized powders of GroupIBIIIA-selenides (such as CuInSe₂ powders), mixtures of Group IB andGroup IIIA binary selenide powders (such as mixtures of Cu_(x)Se andIn₂Se₃ powders), mixtures of Cu-oxide and In-oxide powders, or powdersof ternary oxides (such as Cu—In-Oxide powders). Such techniques,although could provide macro-scale stoichiometric or compositionalcontrol of the film deposited, had several shortcomings. Screen printedlayers utilizing selenide powders, for example, did not yield highefficiency devices due to the poor micro-structure of the layers whichwere difficult to fuse and form dense and large grains during thereaction/annealing step. Oxide powders required high temperatures andextra processing steps to reduce the oxygen content of the resultingcompound film after the reaction step.

Use of metallic particles in the formulation of inks is an attractiveapproach since precursor layers obtained by depositing such inks onsubstrates in the form of thin layers may be readily converted intocompound layers by reaction with S, Se or Te. One of the first attemptsto use metallic powders for CuInSe₂ growth involved; i) mixing of Cupowder and In powder in a suitable liquid carrier, ii) milling thepowders in the liquid carrier to intimately mix them and reduce theirparticle size to form a slurry, iii) depositing the slurry on asubstrate to form a precursor layer, and iv) reacting the precursorlayer with selenium to form CuInSe₂. The quality of films formed by thistechnique was quite poor as evidenced by the poor conversion efficiencyof solar cells fabricated on them.

Indium is a soft material. Therefore, milling of In powder for particlesize reduction is not very efficient. When In particles are milled, theymay actually increase in size due to fusing under the mechanical stressintroduced by the mill such as a ball mill. If In particles are milledtogether with other species, such as Cu particles, they may coat the Cuparticles and also react with them due to the mechanical energy impartedby the mill. In other words, material phase content of each particle maychange as a function of the milling conditions used.

It is known that Ga needs to be included in the compound absorber layerfor the highest efficiency solar cell fabrication. Although about 10%efficient solar cells have been fabricated on CuInSe₂ absorbers, solarcell efficiencies in excess of 16% have been regularly demonstrated onabsorbers containing Ga, and in some cases Ga and S. Such absorberstypically have a Ga/(Ga+In) ratio of 0.20 or larger.

Adding Ga into the composition of metallic powder-based precursors ischallenging. Gallium, like In, is a soft material. It is reactive andalso has a low melting point of about 30° C. Therefore, problemsassociated with addition of indium into the powder-based inkformulations discussed above are even worse for ink formulationscontaining Ga powder. Low melting temperature and reactivity of Ga doesnot allow efficient milling because under the mechanically suppliedenergy by the mill Ga melts and reacts with other species, especiallywith In. If there are Cu particles in the formulation, Ga reacts withthose particles also and coats the Cu particles, causing them to grow insize. Inks with large particle size are not suitable for the formationof compound layers because large particles cause local compositionalvariations in the compound layers formed by reacting them with Group VIAmaterials.

One prior-art technique used for preparing inks comprising metallicparticles involved; i) preparation of Group IBIIIA alloy containingparticles, with Group IBIIIA alloys constituting greater than about 50molar percent of the Group IB elements and greater than about 50 molarpercent of the Group IIIA elements required in the overall composition,ii) milling the powder for size reduction and formation of an ink, iii)deposition of the ink on a substrate to form a precursor layer, and iv)reacting the precursor layer with a Group VIA vapor to convert it into aGroup IBIIIAVIA compound film. Since the above specified Cu—In and Cu—Gaalloy particles are not as soft as In particles or Ga particles, thisapproach reportedly avoided some of the previously discussed problemsassociated with milling In and Ga particles in the presence of Cuparticles. However, milling still changed the phase content of theprecursor ink, and once deposited on the substrate the ink formed porousprecursor layers as reported by G. Norsworthy et al (Solar EnergyMaterials and Solar Cells, vol. 60, p. 127, 2000).

FIGS. 2 and 3 demonstrate the fundamental shortcoming of a method usingmulti-phase metallic particles which are milled for particle sizereduction during an ink preparation step. FIG. 2 shows an exemplarystarting material which is a powder mixture 20. The powder mixture 20comprises Cu—In alloy particles 21 and Cu—Ga alloy particles 22. Let usassume that particle size of the powder mixture 20 is in the order of1000 nm, which is high for ink formulation. To have a target Cu/(In+Ga)ratio of 1.0 and a target Ga/(Ga+In) ratio of 0.3, the Cu—In alloyparticles 21 of FIG. 2 may have a Cu/In ratio of 1.0 and the Cu—Ga alloyparticles 22 may have a Cu/Ga ratio of 1.0. By mixing one mole of Cu—Inalloy particles 21 with about 0.43 moles of Cu—Ga alloy particles 22 thepowder mixture 20 would have the desired overall composition in macroscale.

The alloy particles of the above example may be obtained by varioustechniques. One such approach is melt atomization technique whichinvolves spraying of Cu—In melt or Cu—Ga melt with the desiredcompositions into a container filled with inert gas. Atomized meltdroplets cool down once they leave the atomizing spray nozzle andsolidify to form near-spherical particles with diameters ranging from afew microns to tens of microns. By sieving, the smallest particles maybe separated and used for milling and ink formation.

The Cu—In and Cu—Ga alloy particles formed as described above aremulti-phase particles due to their composition. This can be seen fromthe Cu—In and Cu—Ga binary phase diagrams duplicated in FIGS. 4 a and 4b, respectively. The Cu—In particles of the present example have anoverall compositional ratio of Cu/In=1. This composition is indicated bythe arrow 40 in the replicated Cu—In phase diagram of FIG. 4 a. As canbe seen from FIG. 4 a, there is no stable phase with 50% In in Cu.Therefore, the Cu—In alloy particles 21 of the present example maycontain one or more Cu-rich phases of Cu₁₁In₉, Cu₄In, Cu₇In₄, Cu₉In₄,Cu₁₆In₉, and one or more In-rich phases such as In and CuIn₂. Similarly,the Cu—Ga alloy particles 22 with a composition of 50% Cu and 50% Ga(shown by the arrow 41 in FIG. 4 b) may comprise Cu-rich phases such asCu₉Ga₄, Cu₃Ga₂ and Ga-rich phases such as Ga and CuGa₂. Physicaldistribution of these complex phases within and on each particle isexpected to be quite random and complex. A highly simplifieddemonstration of this fact is shown in FIG. 3. In FIG. 3 exemplary phasecontents of one of the Cu—In alloy particles and one of the Cu—Ga alloyparticles of FIG. 2 are shown. Accordingly, the Cu—In alloy particle 21of FIG. 3 comprises a Cu-rich region 23 and an In-rich region 24. TheCu-rich region may comprise at least one of the Cu-rich phases citedabove (Cu₁₁In₉, Cu₄In, Cu₇In₄, Cu₉In₄, Cu₁₆In₉) and the In-rich regionmay comprise at least one of the In-rich phases such as In and CuIn₂.Similarly, the Cu—Ga alloy particle 22 may comprise Ga-rich regions 25and Cu-rich region 26. The Cu-rich region may comprise one or moreCu-rich phases cited above (Cu₉Ga₄, Cu₃Ga₂) and the Ga-rich region maycomprise one or more Ga-rich phases such as Ga and CuGa₂. As discussedbefore, the phase content and distribution of particles shown in FIG. 3are highly simplified just to demonstrate the point. Actual distributionof phases within each particle is a much more complex, three-dimensionalproblem.

FIG. 3 also shows an exemplary breakage of the Cu—In alloy particle 21and the Cu—Ga alloy particle 22 in smaller pieces after a particlereduction step such as a ball milling step. It should be understood thatthe Cu—In alloy particles 21 and Cu—Ga alloy particles 22 of FIG. 2 maybe mixed with a suitable liquid or carrier before the milling step.Suitable chemical agents such as surfactants, dispersion agents andthickening agents may also be added to this solution, which, aftermilling becomes an ink or dispersion to be deposited on a substratesurface to form a precursor layer. As represented in FIG. 3, after themilling step the particles are broken into smaller pieces. Specifically,in the example of FIG. 3, the Cu—In alloy particle 21 is broken intofour pieces A, B, C and D. The Cu—Ga particle 22 is broken into threepieces E, F and G. As can be appreciated, the smaller particles, A, B,C, D, E, F, and G, which are now in the formulation of the ink all havea different composition and phase content. For example, pieces A and Bare highly In-rich, whereas pieces C and D contain differing amounts ofa Cu-rich phase. Particle E is Cu-rich, whereas pieces F and G containdifferent amounts of Ga-rich phases. When the ink comprising theseparticles of differing compositions is deposited on a substrate in theform of a thin layer and then dried, a precursor film forms. Althoughthe composition of this precursor layer in macro scale is at thetargeted level i.e. Cu/(Ga+In) ratio of 1.0 and Ga/(Ga+In) ratio of 0.3;in micro scale, locally, it may be quite different since the particlesin the ink have differing phase content and composition. If, forexample, in a 10000 nm by 10000 nm area of the film more of theparticles A, B, G and F land, that area may become highly In andGa-rich, whereas a neighboring micro-region may contain more Cu-richparticles and thus have an overall Cu-rich composition. After such aprecursor is reacted with Group VIA materials to form the compoundlayer, such non-uniform areas of the precursor layer are translated intocompound film regions of undesired composition. Solar cells fabricatedon such compound layers have low efficiencies.

As the brief review above demonstrates, there is still need to developlow-cost deposition techniques to form high-quality, dense GroupIBIIIAVIA compound thin films with uniform macro and micro-scalecomposition.

SUMMARY OF THE INVENTION

The present invention is directed to different methods used in theformation of an ink.

The present invention is also directed to the formation of layers usedin the fabrication of a solar cell, particularly the absorber layer.

In another embodiment, the present invention is directed to formulatingan ink, with further embodiments described of using a formulated ink inthe deposition of a precursor layer that will become the absorber layerof the solar cell.

In one embodiment, the invention is directed to formulating an inkcomprising Cu-rich particles and solid Ga—In particles, wherein the stepof formulating is carried out at a temperature such that no liquid phaseis present within the solid Ga—In particles.

In another embodiment, the specific steps taken during the formulationof the ink are described.

In yet another embodiment, the process of using the formulated ink toobtain a precursor layer are described.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and advantages of the present invention, amongothers, will become better understood upon reading the followingdetailed description and upon reference to the drawings where:

FIG. 1 is a cross-sectional view of a solar cell employing a GroupIBIIIAVIA absorber layer.

FIG. 2 shows a prior-art starting material comprising alloy particles.

FIG. 3 shows how the non-uniform phase content of the prior-art alloyparticles can cause non-uniform composition in sub-particles resultingfrom a milling operation.

FIG. 4A is a schematic representation of Cu—In phase diagram.

FIG. 4B is a schematic representation of Cu—Ga phase diagram.

FIG. 5 is a schematic representation of In—Ga phase diagram.

FIG. 6 shows an emulsion formed of melt particles in a liquid.

DETAILED DESCRIPTION

The present invention provides a method that avoids the compositionalnon-uniformities of prior-art methods and also provides approaches toformulate inks or slurries comprising particles of low melting elementsand alloys.

According to a preferred embodiment of the present invention inkscontaining Ga and Ga—In alloys are processed at low temperatures usingparticles of Ga and Ga—In. For example Ga particles that are smallerthan about 200 nm are mixed with Cu and In particles which are alsosmaller than 200 nm in size. The powder mixture is dispersed in acarrier liquid such as water or organic solvent at a temperature lowerthan about 15° C. No milling is carried out for particle reduction.Mechanical agitation or sonication is used to disperse the powders inthe carrier and obtain andispersion. Additives such as surfactants (likesodium lauro sulfate), dispersant agents (available from Rohm and Haas)and thickening agents may be added to the formulation for improving theink or dispersion properties. Although sonication and mechanicalagitation imparts energy and provides heat to the dispersion, coolingmeans are used to keep the temperature below the melting temperature ofGa throughout the ink preparation step to assure that all particles staysolid and they do not fuse together and form agglomerates or largeparticles. Once prepared, the chilled metallic ink comprising the Cu, Inand Ga particles is deposited on the substrate to form a precursor layercomprising metallic particles that are small in size (<200 nm) and havenot reacted with each other since they have been processed at lowtemperatures. This way phase content of each particle is stable from thetime they are added to the ink formulation to the time they aretransferred onto the substrate and form a precursor layer, and theparticle size is well established and small.

In another preferred embodiment In and Ga are introduced in theformulation as a multi-phase alloy. However, unlike in prior arttechnique, each particle of this alloy powder is small (preferablysmaller than about 200 nm in size) and compositionally uniform. Sincethe composition from particle to particle does not change and since theparticles are not milled and broken, inks and precursor layers preparedusing such inks are extremely uniform in both macro-scale andmicro-scale. A specific example of forming the In—Ga particles and inkswill now be described.

A method was described in a parent provisional patent application ownedby the present inventor (Low Cost Deposition of Semiconductor Film,filed Mar. 1, 2004, Ser. No. 60/548,297) where an emulsion of In—Gaparticles was formed in a liquid base and this emulsion was deposited ona substrate to form a precursor layer. FIG. 6 schematically shows suchan emulsion 65 that may be formed by putting In—Ga melt in a liquid 60contained in a container 66. The liquid 60 is kept at above the meltingpoint of the melt. The melt/liquid mixture is mechanically agitatedvigorously to divide the melt into nano size particles 61. Since themelt has a highly uniform composition, each of the melt particles 61also have the same uniform composition in terms of molar ratio ofGa/(Ga+In).

The In—Ga binary phase diagram of FIG. 5 shows that as Ga is added toIn, the melting temperature of Ga—In gets reduced. Let us, as anexample, take the case where the desired Ga/(Ga+In) ratio is 0.4. Toachieve this composition enough Ga and In are weighed separately andmixed. The mix is then heated to form a uniform melt. According to thephase diagram of FIG. 5 the melting temperature of this specificcomposition is about 80° C. Let us assume we keep the melt at 90° C. andthen pour it into the liquid 60 which is also kept at 90° C. There aremany different types of liquids that may be used for this purpose.Silicone based oils among other types of mineral oils may all beeffectively used to disperse the Ga—In melt by mechanical agitation andform an emulsion. For higher Ga compositions with lower melting points,even water based solutions may be effectively used. Point A in FIG. 5represents the composition of a melt particle 61 in the liquid 60 at 90°C. After dispersing the melt and forming nano-size melt particles, thetemperature of the liquid is reduced, reducing the temperature of themelt particles. At a temperature of about 80° C. represented by point Bin FIG. 5, the melt particle starts to form a solid phase. As thetemperature is further reduced to, for example 50° C. (as represented bypoint C) each nano-size particle in the liquid is expected to contain asolid phase S and a liquid phase L. The solid phase, which is an In—Gasolid solution, would have an In-rich composition dictated by point E(about 90% In according to the phase diagram of FIG. 5) and the liquidphase would have a Ga-rich composition shown by point D (about 30% Inaccording to the phase diagram of FIG. 5). As the temperature is lowereddown to room temperature of about 20° C., the composition of the liquidphase would get more Ga-rich (about 85% Ga and 15% In). As should beappreciated particles containing a liquid phase at room temperaturecannot be effectively used in preparation of inks at that temperaturebecause during dispersing the particles with other particles, liquidphases would merge and fuse forming particle agglomerates. Liquid phasewould also easily react with other species and change the phase contentof the particles. Accordingly, the present invention provides forcooling the temperature of the liquid 60 to below about 15° C. where aneutectic point W exists between Ga and In. Once the temperature islowered below 15° C. all particles solidify and they all contain thesame amount of In and Ga given by the fixed ratio of Ga/(Ga+In) in theoriginal melt and their phase content is very similar (an In rich solidphase and a Ga-rich solid phase). It should be noted that any Ga—Incomposition, unless it is within the solid solution region S to theright of the phase diagram in FIG. 5 would contain a liquid phase unlessits temperature is lowered to or below the temperature at the eutecticpoint W. This temperature is around 15° C. Particles prepared by thisapproach have sizes smaller than 200 nm and preferably smaller than 100nm. They are washed out of the solution and cleaned using solventschilled to temperatures lower than 15° C. Then the In—Ga particles ofuniform composition are mixed by Cu powder at the desired stoichiometricCu/(Ga+In) ratio such as a ratio in the range of 0.7-1.0 and the mixtureis used to form a highly uniform ink. The Ga/(Ga+In) molar ratio may bein the range of 0.2-0.9, prefereably in the range of 0.3-0.7. It shouldbe noted that all these process steps have to be carried out at lowtemperature, preferably below about 15° C., to avoid formation of aliquid phase in the Ga—In particles. Once the ink comprising Cuparticles and Ga—In particles is deposited on a substrate to form aprecursor layer, the precursor layer may be heated up to roomtemperature or above room temperature. At this time after the precursorlayer formation it is beneficial to allow formation of a liquid phase inor on the surface of the In—Ga particles. Such liquid phase helps fusethe particles together and helps form a dense precursor film which maythan be reacted with Group VIA elements to form a dense andcompositionally uniform compound layer.

It should be noted that one attractive feature of the present inventionis the low process temperatures used in powder formation, inkformulation and ink deposition. This lowers the cost of processing andsimplifies manufacturing. Low melting temperatures of Ga and Ga—Inalloys, which present challenges in prior art techniques are actuallyused to the benefit of the processing engineer in the present invention.The processing of the present invention may be carried out at atemperature range of −5 to 20° C., preferably in the 0 to 15° C. range.

Although the preferred composition of the ink comprises Cu particles andIn—Ga particles, it is possible to use Cu—In particles (in the form ofCu—In alloy particles or Cu—In solid solution particles) and/or Cu—Gaparticles (in the form of Cu—Ga alloy particles or Cu—Ga solid solutionparticles) instead of or in addition to the Cu particles. Furthermore,In particles may also be added to the overall formulation. The particlesize for all powders used is smaller than 200 nm, preferably smallerthan 100 nm. Although the phase distribution within the Ga—In particlesis not critical for the present invention, the simple phase diagram ofIn—Ga assures that within each particle there would be a solid phase(which is a solid solution of Ga in In) and a Ga-rich phase which isinitially a liquid and then is solidified during cooling down. If thecooling ramp during the near-spherical Ga—In nano-particle formation iscontrolled (slow cooling of 1-10° C. per minute) it is possible to haveeach particle contain the In-rich phase at the core of the spheres andthe Ga-rich phase at the surface. This is a preferred phase distributionwithin each particle, because after deposition of the ink and formationof a precursor layer, a dense structure is formed comprising thesespherical particles. Upon heating to room temperature the Ga-rich phaseon the surface of the particles melts and fuses with neighboringparticle surfaces forming a fused layer that is not powdery any more.

After the preparation of an ink using the powder mixtures of the presentinvention a precursor layer may be deposited by various means such asdoctor blading, gravure deposition, spin coating, dip coating, rollcoating and spraying. In one embodiment the precursor layer whichcomprises Cu, In and Ga (Group III material provided either as In powderand Ga powder or in the form of Ga—In powder or a mixture of both) isexposed to Group VIA element(s) at elevated temperatures. Thesetechniques are well known in the field and they involve heating theprecursor layer to a temperature range of 350-600° C. in the presence ofat least one of Se vapors, S vapors, and Te vapors provided by sourcessuch as solid Se, solid S, solid Te, H₂Se gas, H₂S gas etc. In anotherembodiment a layer or multi layers of Group VIA materials are depositedon the precursor layer and the stacked layers are then heated up in afurnace or in a rapid thermal annealing furnace and like. Group VIAmaterials may be evaporated on, sputtered on or plated on the precursorlayer. Alternately inks comprising Group VIA nano particles may beprepared and these inks may be deposited on the precursor layers to forma Group VIA material layer comprising Group VIA nano particles.

Reaction may be carried out at elevated temperatures of 350-600° C. fortimes ranging from 1 minute to 30 minutes depending upon thetemperature. As a result of reaction, the Group IBIIIAVIA compound isformed from the precursor.

Solar cells are completed using materials and methods known in thefield. For example a thin (<0.1 microns) CdS layer may be deposited onthe surface of the compound layer using the chemical dip method. Atransparent window of ZnO may be deposited over the CdS layer usingMOCVD or sputtering techniques. A metallic finger pattern is optionallydeposited over the ZnO to complete the solar cell.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art, and from the scope of the claims set forth below.

1. A method comprising: formulating an ink comprising Cu-rich particlesand solid Ga—In particles, wherein the step of formulating is carriedout at a temperature such that no liquid phase is present within thesolid Ga—In particles.
 2. The method of claim 1 further including thestep of depositing the ink on a substrate to form a precursor layer forthe growth of a Cu(In,Ga)(VIA)₂ compound film absorber of a solar cell3. The method according to claim 1 wherein the largest dimension of theCu-rich particles and the solid Ga-In particles is less than 200 nm. 4.The method of claim 2 wherein the temperature during the step offormulating is at or below 15° C.
 5. The method of claim 2 wherein thetemperature during the step of depositing is at or below 15° C.
 6. Themethod of claim 2 wherein the Cu-rich particles comprise at least one ofCu particles, Cu—In solid solution particles, Cu—In alloy particles,Cu—Ga solid solution particles and Cu—Ga alloy particles.
 7. The methodof claim 2, wherein the ink further comprises at least one of Gaparticles and In particles.
 8. The method of claim 2 further comprisingthe step of heating the precursor layer to above 15 C to form a fusedprecursor layer.
 9. The method of claim 8 further comprising the step ofreacting the fused precursor layer with a Group VIA material to form theCu(In,Ga) (VIA)₂ compound film.
 10. The method of claim 9 wherein theGroup VIA material is at least one of Se and S and the step of reactingis carried out at a temperature of 350-600 C.
 11. The method of claim 10further comprising the step of depositing a transparent layer on theCu(In,Ga) (VIA)₂ compound film to fabricate the solar cell.
 12. Themethod according to claim 1 wherein the step of formulating includes thesteps of: placing solid Ga—In particles and Cu-rich particles into acarrier liquid having a temperature at or below 15° C., thereby forminga mixture, adding into the mixture at least one dispersion forming agentto form a second mixture and agitating the second mixture to form adispersion while keeping the temperature of the carrier liquid at orbelow 15 C.
 13. The method of claim 12 wherein the amount of Cu-richparticles and Ga—In particles placed into the carrier liquid is selectedto obtain a pre-determined Cu/(In+Ga) molar ratio and a pre-determinedGa/(Ga+In) ratio in the second mixture.
 14. The method of claim 13wherein the predetermined Cu/(In+Ga) molar ratio is in the range of0.7-1.0 and the predetermined Ga/(Ga+In) molar ratio is in the range of0.2-0.8.
 15. The method of claim 12 wherein the carrier liquid is water.16. The method of claim 12 further including the step of forming whereinthe solid Ga—In particles used in the step of placing, the step offorming the solid Ga—In particles are obtained by a method comprising;forming a Ga—In melt with a Ga/(Ga+In) molar ratio and a pre-determinedmelting temperature, adding the Ga—In melt in a liquid heated to atleast to the pre-determined melting temperature, agitating the Ga—Inmelt and the liquid to form an emulsion comprising nano-size Ga—In meltparticles, lowering the temperature of the emulsion to below 15 C tosolidify the nano-size Ga—In melt particles and to thereby form solidGa—In particles, and removing the solid Ga—In particles.
 17. The methodof claim 16 wherein the Ga/(Ga+In) molar ratio is in the range of0.2-0.8.
 18. The method of claim 12 wherein the Cu-rich particlescomprises at least one of Cu particles, Cu—In solid solution particles,Cu—In alloy particles, Cu—Ga solid solution particles and Cu—Ga alloyparticles.
 19. The method according to claim 1 wherein, prior to thestep of formulating the ink, there is included the step of forming apowder mixture comprising Group IB-rich particles with a first phasecontent and Group IIIA-rich particles with a second phase content,wherein the largest dimension of the Group IB-rich particles and theGroup IIIA-rich particles is less than 200 nm, wherein the Cu-richparticles and the solid Ga—In particles used in the step of formulatingthe ink are obtained from the powder mixture, and wherein the powdermixture is maintained at a temperature at or below 15 C, and after thestep of formulating the ink, further comprising the step of depositingthe ink on a substrate to form a precursor layer comprising GroupIB-rich particles with the first phase content and Group IIIA-richparticles with the second phase content.
 20. The method of claim 19wherein the Group IB-rich particles comprise at least one of Cuparticles, Cu—In alloy particles, Cu—In solid solution particles, Cu—Gaalloy particles and Cu—Ga solid solution particles.
 21. The method ofclaim 20 wherein the Group IIIA-rich particles comprise at least one ofGa particles, In particles, Ga—In solid solution particles and Ga—Inalloy particles.
 22. The method of claim 19 wherein the step ofdepositing is carried out at or below 15 C.
 23. The method of claim 22further comprising the step of heating the precursor layer to form afused precursor layer.
 24. The method of claim 23 further comprising thestep of reacting the fused precursor layer with a Group VIA material toform a Group IBIIIAVIA compound film.